This article is the eighth in our latest series, The 12 Rhythms of Christmas, where each day we examine a new rhythm disorder. It’s a continuation of the theme behind last year’s 12 Leads of Christmas.
In our recent articles we’ve discussed three different types of AV-block that cause dropped P-waves: type I, type II, and 2:1 AV-block. Consider the prototypical tracings from each article:
Each shows different conduction ratios—ranging from 2:1 to 3:2 to 4:3—but they don’t demonstrate something like 3:1 conduction, where two P-waves in a row would be blocked before the third one conducts.
That’s because we classify that with another distinct term:
High-grade AV-block (sometimes called advanced AV-block) is how we describe a form of pathological AV-block where two or more consecutive P-waves fail to conduct to the ventricles.
Why don’t we just call the tracing in Fig. 4 type II AV-block? Think back to the basis of our article on 2:1 AV-block: Since we never see two P-waves in a row that conduct, we cannot assess whether the PR-interval is progressively increasing (as in type I AV-block) or fixed (as in type II AV-block). You might think that all high-grade AV-blocks must be due to a type II mechanism because the conduction defect looks so severe, but even type I AV-blocks can exhibit the sort of behavior we see above. In fact, based on subsequent tracings (not shown here), there’s a pretty decent chance both the patients whose rhythms we’re going to examine in this post were experiencing high-grade AV-block due to an underlying type I mechanism.
So, just like 2:1 AV-block, high-grade AV-block can have an underlying conduction defect that is either type I or type II in mechanism, but since we usually cannot tell which it is from the surface ECG, we have to use a different name and reserve those latter two terms for cases where we can prove (or at least highly suspect) the actual pathology at play.
High-grade AV-block does not include physiological AV-nodal activity, like we see sometimes with ectopic atrial tachycardia or atrial flutter, where it’s possible to see two consecutive non-sinus P-waves dropped as a matter of normal physiology. As a result, to be diagnosed as “high-grade AV-block” the atrial rate should be reasonable (Marriott proposed less than 135 /min) and regular. Excessive atrial rates, like the 300 /min we see with atrial flutter, can display 3:1 or 4:1 conduction as a normal finding—especially if the patient is on an AV-nodal blocking medication.
It also does not include complete heart block, where no sinus P-waves are conducted to the ventricles.
Importantly, high-grade AV-block is not synonymous with high-degree AV-block. The latter is a non-specific term sometimes used to designate more malignant conduction disturbances—like type II and complete AV-block—from first degree AV-block (and sometimes type I), but I’ve never seen a consensus for its definition and its use is not encouraged.
Most resources do not give much consideration to high-grade AV-block, which is a shame because it can present some pretty interesting rhythms. Let’s look at a few, starting with the 12-lead from Fig. 4.
I would not fault most folks for wanting to call the rhythm in Fig. 7 complete AV-block—every single provider involved in the patient’s care did—but it’s not. The first clue here is the fixed PR-intervals:
That could just be a fluke of timing that is sometimes seen with complete heart block, so let’s also look at the RR-intervals.
Now that’s an important finding; there is subtle but significant variation in the RR-intervals. In order for the PR-intervals to be perfectly fixed with a fluctuating ventricular rate, there must be a corresponding rise and fall in the atrial rate as well. For those two things to occur independently—and with the exact same magnitude—pushes this beyond the realm of coincidence.
There must be communication between the atria and ventricles, and we can say with certainty that we are looking at high-grade AV-block.
Here’s a more complicated (and interesting) example:
The first clue is that we’re not dealing with complete AV-block is that the ventricular rate is again irregular—this time markedly so.
Most examples of complete AV-block display an unwavering ventricular rate since the ventricles are under the sole control of an isolated junctional or ventricular escape pacemaker, which are both typically very regular (see Fig. 6). Even if an observant reader notes the irregularity in the above rhythm, their next step is often to mistakenly attribute it to PVC’s. While the early complexes are indeed wide (I peg them right around 125 ms), they do not display a morphology really consistent with ventricular ectopy. I’d describe them more as resembling a left anterior fascicular block with non-specific QRS widening.
It turns out the early complexes we see above are actually capture beats from occasional sinus impulses managing to make it through the AV-node. Since some sinus discharges are making it through, it cannot be complete AV-block.
Proof that the early beats really are due to sinus capture can be found in the PR-intervals (again).
You will note that the early capture beats display a fixed PR-interval of 425 ms (shown in red). I attempted to measure a possible PR-interval for the late QRS complexes but: 1) it doesn’t make sense for them to show a longer PR-interval than the early QRS’s (the immediately adjacent P-waves produce too-short a PRi to be viable options), and 2) there is a bit of variance. This variance is pretty small and wouldn’t be a big issue if not for one final scrap of evidence: Looking back at Fig. 11, the escape complexes share the exact same “escape interval” (a term I think I just made up) of 1730 ms. If the late complexes were actually sinus is origin it be highly unlikely for this perfect coincidence to occur, especially since there is some minor irregularity in the atrial rate.
It might seem counter-intuitive for the escape complexes to appear more narrow than the capture beats, but that is because they arise from the His-Purkinje system below the level of the block, while the capture beats—which have to traverse the block—also experience a certain degree of non-specific aberrancy en route to the ventricles.
Here’s an even more subtle example of the same phenomenon, but hopefully it will reinforce the concepts we just discussed. It’s actually from the same patient as Fig. 7, just from two days before during her initial presentation to the ED.
The ventricular rate appears more regular in this example, but pay close attention to the shape of the QRS complexes in lead II. They appear to alternate in morphology!
This subtle change in QRS morphology prompts us to look for more clues…
As in the last case, the longer RR-intervals are perfectly fixed while there is a minor variation in the shorter RR-interval. What are the PR-intervals up to?
Again, the slightly early complexes all share the same exact PR-interval while there are subtle variations when you attempt to couple P-waves to the late-arriving QRS complexes. It seems we are again seeing high-grade AV-block with alternating escape and capture beats. This subtle form of alternating long and short RR-intervals due to alternating escape and capture beats (respectively) can be termed, appropriately enough, escape-capture bigeminy.
Tomorrow we’ll discuss the dramatic situation we see below, where most of the time the patient is in a normal sinus rhythm with no significant AV-block but then he or she suddenly drops several P-waves in a row before just as suddenly resuming normal conduction.
For more on high-grade AV-block and to see more tracings from the patient featured in Fig. 7 and Fig. 14, please check out CCT Jason Roediger’s post and ladder diagrams walking through why we see the patterns we do with high-grade AV-block; they are phenomenal! Here’s a link, and make sure you check out the comments at the bottom.
Check out the rest of The 12 Rhythms of Christmas! (updated as new posts come out):
The 12 Rhythms of Christmas: Sinus Tachycardia
The 12 Rhythms of Christmas: Sinus Bradycardia
The 12 Rhythms of Christmas: Atrial Flutter
The 12 Rhythms of Christmas: First Degree AV-Block
The 12 Rhythms of Christmas: Type I AV-Block
The 12 Rhythms of Christmas: Type II AV-Block
The 12 Rhythms of Christmas: 2:1 AV-Block